As a rule, self-propelled working machines, particularly harvesting machines, are driven hydrostatically. Normally a transmission that can be shifted through a number of gear ratios is inserted between the axle drive and the hydraulic motor. In the state of the art, these transmissions are shifted manually. In the working or harvesting operation, as a rule, only a few of the available gear ratios are required (the first and/or second gear), While during the transport of the working machine or operating on public roads, the normal operation is faster so that the other gear ratios are needed. A particular disadvantage lies in the fact that shifted transmissions for harvesting machines require that the machine be brought to a complete stop before a gear shift can be performed. With normal motor vehicles operating on roads, synchronized transmissions are used in order to have the speed of one transmission shaft conform to the speed of another, so as to perform smooth gear shifts. In harvesting machines, the synchronization is not used for conformity of velocity, but for the conformity of the position or the phase in order to provide the assurance that the gear teeth that are to be brought into mesh are properly aligned with each other. Much time is lost by the stopping and restarting, and thereby dangerous situations can develop, particularly in traffic on the road, in case a gear shift becomes necessary.
EP 0 819 562 A proposes an electronic control arrangement that automatically brings the harvesting machine to a standstill after a gear change input of an operator and then shifts the gears. While a shift lever may be saved and the operator relieved of the task of shifting, it is nevertheless necessary to bring the machine to a standstill before the shifting process, which has the aforementioned disadvantages.
EP 1 052 388 A proposes a harvesting machine in which the rotational speed of the drive engine is controlled as a function of an automatically recognized operating mode. During harvesting, the operation is performed at a fixed rotational speed, while during transport operations or operation on public roads, the rotational speed of the engine is a function of the immediate velocity. Here, too, the immediate gear ratio is selected manually.
The problem underlying the invention is seen in the need to make available an improved drive system for a self-propelled working machine.